Metasurface antennas have recently emerged as a new technology for generating steered, directive beams from a lightweight, low-cost, and planar physical platform. Such metasurface antennas have been recently used for microwave applications and stand to provide relevant hardware and software gains for various applications such as microwave imaging, communications and synthetic aperture radar.
Metasurface antennas may comprise a waveguide structure, loaded with resonant, complementary metamaterial elements in the upper surface of the waveguide that can selectively couple energy away from a guided wave into free space as radiation. By tuning the constituent elements' characteristics, a hologram at the aperture plane can be achieved, in which the waveguide mode acts as the reference wave and the collection of tuned elements form the hologram. The overall radiation from these holographic antennas can thereby be modulated to form arbitrary patterns by the use of electronic tuning. These antennas are capable of achieving comparable performance to phased array antennas from an inexpensive and easy-to-manufacture hardware platform.
By using simpler elements as compared to phased arrays, the operation of metasurfaces is easier and faster. These elements, however, do not exhibit the same level of control as is achievable with phase shifters and amplifiers, common to phased array architectures. To regain some of the control possible with phased array elements, metasurface elements are typically spaced closer together in order to more finely sample the guided wave. In further contrast to phased array antennas, however, tuning metamaterial elements does not provide independent control of both the magnitude and phase of each individual element in the array. Instead, tuning a metamaterial unit cell results in a shift in the resonant frequency, which shifts both the magnitude and phase response with only one control knob. As a result, attempting to create arbitrary magnitude or phase patterns within a metasurface antenna can yield undesirable results.
The reason that creating arbitrary magnitude or phase patterns does not work as intended can be traced back to the coupled nature of the magnitude and phase of the resonant elements in a metasurface antenna. Considering a phase pattern, when tuning an antenna element to a certain phase value, the magnitude of the element becomes correspondingly shifted. This arbitrary shift can lead to unwanted periodic behavior or to low radiation efficiency. In the same manner, similar side effects of phase artifacts result when attempting to create a magnitude pattern. These problems do not exist in traditional phased array systems because amplifiers and phase shifters can directly compensate for any such unwanted artifacts.
One method of modulating individual elements in a metasurface antenna element is to turn some elements “on” while others are kept “off”. This is referred to as binary modulation. Another method that is used is referred to as greyshade modulation in which antenna elements have more states than simply one “on” states and an “off” state.